Non-Metallic Flow-Through Electrodeless Conductivity Sensor and Leak Detector

ABSTRACT

A non metallic flow through electrodeless conductivity sensor is provided with a conduit having primary and secondary process fluid flowpaths to form a fluid loop. At least one drive and one sense toroid surround the conduit on the fluid loop. Voltage supplied to the drive toroid induces a current in the sense toroid via the fluid loop to eliminate any need for metallic electrodes in contact with the process fluid. At least one additional drive and/or sense toroid is disposed on the fluid loop to enhance induction. Optionally one or more sense coils are disposed about the conduit outside of the fluid loop to cancel out stray electrical noise. An optional conductor disposed along the conduit detects any fluid leakage through changes in resistance thereof.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/351,856 entitled Non-Metallic Flow-Through Electrodeless ConductivitySensor and Leak Detector, filed on Feb. 9, 2006 which claims priority toU.S. Provisional Patent Application Ser. No. 60/676,765 entitledNon-Metallic Flow Through Electrodeless Conductivity Sensor, filed onMay 2, 2005.

TECHNICAL FIELD

This invention relates to conductivity sensors and more particularly toelectrodeless conductivity sensors configured to detect the conductivityof process fluid flowing through a conduit.

BACKGROUND INFORMATION

Throughout this application, various publications, patents and publishedpatent applications are referred to by an identifying citation. Thedisclosures of the publications, patents and published patentapplications referenced in this application are hereby incorporated byreference into the present disclosure.

Conductivity measurements of a chemical solution may be made by applyinga voltage across a pair of electrodes and immersing them in thesolution. The electric current passing through the system isproportional to the conductivity of the solution. This technique,however, is not optimal if the solution to be measured is chemicallyincompatible with the metallic electrodes, e.g., resulting in chemicalattack or contamination of the solution and/or electrodes.

Another approach involves an electrodeless toroidal conductivitymeasurement. In this approach, an electric transformer is effectivelycreated through the use of driver and sensor toroidal coils surroundinga ‘core’ formed at least partially by the solution under test. Thetoroids are typically disposed within an electrically insulative,magnetically transparent housing having a fluid flow path which passesaxially therethrough. The driver is supplied with a voltage whichinduces an electromagnetic field in the solution passing through theflow path, which then induces a current in the sense coil. The inducedcurrent is proportional to the conductivity of the solution beingmeasured.

An example of such a toroidal conductivity sensor is disclosed in Reese,U.S. Pat. No. 5,157,332. A commercial example of a similar sensor isknown as the 871EC™ invasive conductivity sensor available from InvensysSystems, Inc. (Foxboro, Mass.). As shown in FIG. 1, a section of such anelectrodeless conductivity sensor 20 includes toroidal coils 11, 12, 13encased in a housing 21, which may be immersed in the fluid to bemeasured. The housing 21 defines a central bore 19 which allows fluid topass axially through the toroids 11, 12, 13, without contacting them.The induction loop of the ‘core’ is completed by the process solutionwithin which the sensor is immersed.

Where a fluid to be measured is flowing through a conduit, it may not bepossible or desirable to immerse a sensor in the fluid. In this event,driver and sensor toroidal coils may surround a pipe carrying theliquid. A commercial example of such a sensor is known as the 871FT™(Invensys Systems, Inc.). However, in order for induction to occur, anelectrical loop must be completed outside the coils, typically byclamping a metallic strap to metallic portions of the pipe upstream anddownstream of the toroids. A drawback of this approach, however, is thatmetallic pipe portions cannot be used when the process fluid attacks oris otherwise incompatible with metals.

In an alternate approach, the induction loop may be completed by thefluid itself, by providing a secondary flow path that bypasses one ormore of the toroids. An example of such a fluid loop is disclosed inU.S. Pat. No. 2,709,785 to Fielden. A drawback of this approach is thatthe limited cross section, relatively long length and high resistance ofthe fluid itself, adds a net resistance to the induced current whichtends to adversely affect the sensitivity of conductivity measurement.Approaches intended to enhance the sensitivity of conductivity sensorsinclude that disclosed by Ogawa, in U.S. Pat. No. 4,740,755. Ogawadiscloses toroids on a fluid loop with dimensions calculated to “providea low value for the ratio of the length of fluid flow loop . . . to thecross sectional area of the flow path, which in turn provides goodsensitivity.” (Ogawa col. 2 lines 42-47). A drawback to this approach isthat Ogawa's toroids are taught to be coplanar and physically separatedin order to reduce leakage coupling between the transformers. (Ogawa atcol. 1, lines 34-38, col. 2 lines 47-52, col. 4, lines 49-55).

A need therefore exists for an electrodeless conductivity measurementsystem that addresses one or more of the aforementioned drawbacks.

SUMMARY

In accordance with one aspect of the invention, an electrodelessconductivity sensor is provided for determining conductivity of aprocess fluid. The sensor includes a non-metallic conduit which divergesdownstream of an inlet into first and second legs, and re-convergesupstream of an outlet, to form a fluid-flow loop between the inlet andthe outlet. First and second toroids, each configured as either a driveor a sense coil, are disposed about one of the first and second legs. Athird toroid configured as either a redundant drive or sense coil isalso disposed about one of the legs. A connector is configured to couplethe first, second and third toroids to an analyzer.

In another aspect of the invention, an electrodeless conductivity sensorincludes a non-electrically conductive fluid flow conduit which divergesdownstream of an inlet into first and second legs, and then re-convergesupstream of the outlet to form a fluid loop between the inlet and theoutlet. A housing encloses the legs. Toroids configured as first andsecond type coils are disposed about the legs. The first and second typecoils are selected from the group consisting of drive coils and sensecoils. A toroid of the first type is disposed between toroids of thesecond type on each of the legs. In addition, at least one other toroidconfigured as a sensor coil is disposed about the conduit outside of thefluid loop. Shields are interspersed between the coils to magneticallyisolate the coils from one another. A calibration loop including anelectrical conductor extends through the toroids on the two legs, and aleakage detector including an other electrical conductor is disposedwithin the housing in spaced relation from the toroids. The leakagedetector is connectable to resistance measuring means.

A further aspect of the invention includes an apparatus for detectingleakage of process fluid from a fluid flow conduit. The apparatusincludes an electrical conductor disposed in leakage-contacting relationto the conduit, the conductor having a predetermined electricalresistance. A test port has terminals coupled to opposite ends of theconductor, and is couplable to resistance measuring means for measuringresistance of the sensing conductor.

Yet another aspect of the invention includes a method for fabricating asensor for detecting conductivity of a fluid flowing through a conduit.The method includes providing a non-metallic conduit for the flow of aprocess fluid, diverging the conduit downstream of an inlet into firstand second legs, and re-converging the legs upstream of an outlet toform a fluid-flow loop between the inlet and the outlet. The method alsoincludes placing a drive toroid about one of the legs, placing a sensetoroid about one of the legs, and placing a redundant drive or sensetoroid about one of the legs. A connector is configured to couple thetoroids to an analyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of this invention will bemore readily apparent from a reading of the following detaileddescription of various aspects of the invention taken in conjunctionwith the accompanying drawings, in which:

FIG. 1 is a cross sectional elevational view of a portion of an ECsensor of the prior art;

FIG. 2 is an elevational view of an embodiment of the claimed invention,with optional features shown in phantom;

FIG. 3 is an exploded view, with portions shown in phantom, of theembodiment of FIG. 2;

FIG. 4 is a partially cross-sectional elevational view of an alternateembodiment of the claimed invention, with optional portions thereofshown in phantom;

FIG. 5 is a plan view of the embodiment of FIG. 4;

FIG. 6 is an exemplary wiring schematic of an embodiment of the presentinvention; and

FIG. 7 is an exemplary wiring schematic of an alternate embodiment ofthe present invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration, specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that other embodiments may be utilized. It is also to beunderstood that structural, procedural and system changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims and their equivalents. For clarity of exposition, likefeatures shown in the accompanying drawings are indicated with likereference numerals and similar features as shown in alternateembodiments in the drawings are indicated with similar referencenumerals. Where used in this disclosure, the term “axial” when used inconnection with an element described herein, shall refer to a directionparallel to the flowpath and/or downstream flow of the process solutiontherethrough.

In a representative embodiment of the present invention, a fluid to bemeasured flows through a conduit fabricated from a non electricallyconductive material. Toroidal coils surround the conduit, withoutphysically contacting the fluid. A voltage is supplied to a driver coil,which induces a magnetic field in the fluid flowing within the conduit.This magnetic field similarly induces an electric current in a sensorcoil.

A complete loop through which the magnetic field propagates is formed bythe fluid itself, via a secondary flowpath which diverges from theprimary flowpath of the conduit upstream from the measuring toroidalcoils, and reconverges with the primary flowpath of the conduitdownstream from the measuring coils. The toroidal coils may be disposedon the primary flowpath, the secondary flowpath, or both.

The instant inventors have recognized that the sensitivity of theconductivity measurement tends to be adversely affected by the distancethe magnetic field must travel through the fluid loop. To compensate forthis, embodiments of the invention have been provided with one or moreredundant toroidal coils, wired in parallel, to boost induction.

Particular embodiments may also include additional sensor coils disposedupstream and/or downstream of the fluid loop. These additional sensorcoils may be wired in reverse phase relative to the driver coils tocancel out stray electrical noise in the system. In addition, a leakdetector conductor may optionally be disposed in proximity to theconduit. This conductor may be fabricated from a material sensitive tothe process fluid, and may be helically coiled around the conduit, orsimply supported parallel to thereto. The conductor may then beconnected to an Ohmmeter, whereupon any change from a known baselineresistance, such as may occur due to chemical attack by the processfluid, would be indicative of a leak in the conduit.

Turning now to the figures, an embodiment of the present inventionincludes conductivity sensor 200 as shown in FIG. 2. Process fluid flowsthrough conduit 202 in a downstream direction from an inlet 204 to anoutlet 206. The conduit diverges at point 208 and forms two flow paths,the primary flow path 210 and the secondary flow path 212. The conduitthen reconverges at point 209. The primary flow path 210 and secondaryflow path 212 form a fluid-flow loop 214.

In this embodiment, toroids 220, 222, and 224 are located on the primaryflow path 210. As described hereinabove, these toroids 220, 222, 224surround conduit 210 and are physically and electrically isolated fromthe process fluid flowing through conduit 210. In one embodiment, thecentral toroid 222 is a sense toroid, and the outer toroids 220, 224 aredrive toroids. In another embodiment, the central toroid 222 is a drivetoroid, and the outer toroids 220, 224 are sense toroids.

For ease of explanation, the outer toroids 220 and 224 will bedesignated as drive toroids, and the central toroid 222 will bedesignated as a sense toroid, with the understanding that the followingdiscussion may also be applicable to the opposite configuration in whichthe drive and sense toroids are reversed. Electric current supplied tothe redundant driver toroids 220, 224 creates a magnetic field whichinduces an EM field or current which flows through fluid loop (core)214. This induction similarly induces a current in sense toroid 222,which is proportional to the conductivity of the process fluid.

Use of primary and secondary flow paths 210 and 212 enables theinduction loop to be formed by the fluid itself, rather than via ametallic strap as commonly used in the prior art. This enables sensor200 to measure the conductivity of fluids that tend to attack or areotherwise incompatible with metallic fittings or conductors. Moreover,the use of redundant toroids (either as a drive or sense toroid) asshown, provides enhanced sensitivity which compensates for the adverseaffects on sensitivity otherwise associated with relatively highresistance fluid-loop inductive cores.

Optionally, embodiments of the invention may include one or moreadditional toroids 230, 232, and 234 (shown in phantom) located alongfluid loop 214. For convenience, these additional toroids are shown asdisposed on secondary flow path 212, but may be substantially anywherealong loop 214. While nominally any combination of drive and sensetoroids may be used, in a representative embodiment, toroids 230 and 234may be operated as drive toroids, with toroid 232 as a sense toroid.These additional toroids may be used in combination, e.g., by wiringthem electrically in parallel with respective ones of toroids 220, 222and/or 224, to further enhance the induction via fluid loop 214.

In another variation of the instant invention, one or more additionalsensor toroidal coils 240, 242 may be disposed upstream and/ordownstream of fluid loop 214. These sensor coils 240, 242 may be wiredin reverse phase with the other (on-loop) sense coils 222, 232, etc., toeffectively cancel out electrical noise which may be present in theconduit 210 outside fluid loop 214.

Turning now to FIG. 3, one set of three toroids, e.g., toroids 220, 222and 224, is shown in an exploded view. As shown, toroids 220 and 224 maybe connected in parallel to a source of electric current via cables 360,364, to function as drive toroids. Toroid 322 is connected by cable 362to a conventional analysis apparatus, such as the 875EC Series Analyzersor 870ITEC Series Transmitters (Invensys Systems Inc., Foxboro, Mass.)which may be further coupled to a conventional factory automationsystem.

As also shown, shields 350, 352, may be interspersed between the toroidsto help prevent the fields generated by the drive toroids frominterfering with one another and/or with the sense toroids. In desiredembodiments, these magnetic shields 350, 352 extend circumferentiallyabout conduit 302, while remaining physically and electrically isolatedfrom the process fluid flowing therethrough. For example, in particularembodiments magnetic shields 350, 352 are centrally apertured discs, inthe form of copper washers. Ground wire 354 connects shields 350, 352 toone another, and to ground.

Referring now to FIGS. 4 & 5, any of the aforementioned embodiments maybe disposed within a housing 469, to form an enclosed conductivitymeasurement device shown at 400. In this embodiment, driver toroids 420,424 and sense toroid 422 are coupled to a modular connector portion 470to facilitate removable connection to a transmitter or other datacapture/calculation device or system. Connector portion 470 may benominally any connector type known to those skilled in the art. A testport 476 is also shown, which may be coupled to opposite ends of acalibration conductor 471 of known resistance, which forms a looppassing through the toroids as shown. Calibration conductor 471 may beused to calibrate device 400 by shorting the ends thereof (e.g., using acalibrator plugged into test port 476), and then operating the devicewithout process fluid in fluid loop 214. The output of the sensortoroids may then be calibrated to match the known resistance ofconductor 471, as will be discussed in greater detail hereinbelow. Thoseskilled in the art should recognize that this calibrationport/conductor, and any other aspects shown and described with respectto a particular embodiment, may be applied to any other of theembodiments described herein, without departing from the spirit andscope of the present invention.

As also shown, an optional leak detection conductor 477 (shown inphantom) may be provided. The conductor 477 may be disposed atsubstantially any location likely to come into contact with processfluid leaking from conduit 402. In the embodiment shown, conductor 477may be disposed at any convenient location within housing 469, such asat the lowest installed location thereof, i.e., at the point at whichany leaked process fluid would collect. In addition, or alternatively,conductor 477 may be extended alongside, or wrapped helically aroundconduit 402 as shown in phantom. This latter approach may beparticularly useful in embodiments not having a housing 469.

Conductor 477 may be fabricated from a material sensitive to theparticular process fluid under test. For example, since many of theembodiments described herein are intended to measure the conductivity ofprocess fluids such as caustic acids (e.g., HF, HCl) that chemicallyattack various types of metals (e.g., aluminum), conductor 477 may befabricated from such a metal. The resistance of conductor 477 may thenbe monitored, e.g., via terminals C & D (FIG. 6) of test port 476, tomeasure any changes in resistance which may be indicative of fluidhaving leaked from conduit 402 and contacted conductor 477. For example,an increase in measured resistance may occur due to chemical attack andan associated reduction in cross-sectional area of the conductor 477.

As a further option, conductor 477 may also include a discrete resistor478 (shown in phantom) as desired to customize the baseline resistance.A resistor 478 may be chosen to increase the baseline resistance beyondthe expected resistance of the process fluid. Contact with any leakedprocess fluid of lower resistance would tend to decrease the measuredresistance at test port 476, to indicate the presence of the leak. Thisconfiguration may be particularly useful when measuring a process fluidthat does not chemically attack conductor 477, but is neverthelessincompatible with metals, such as due to contamination/purity concerns.

Although leak detection conductor 477 and optional resistor 478 areshown and described as incorporated within the various conductivitysensors of the present invention, those skilled in the art shouldrecognize that it may be used independently and/or in combination withnominally any type of fluid sensor, without departing from the spiritand scope of the present invention. For example, leak detectionconductor 477 and/or resistor 478 may be incorporated with varioustemperature detectors, pressure detectors, conductivity sensors, pHsensors, ORP sensors, flow meters, and combinations thereof. Commercialexamples of such devices include the 83 Series Vortex Flowmeters, I/ASeries Pressure Transmitters, 134 Series Intelligent DisplacementTransmitters, I/A Series Temperature Transmitters, 873 SeriesElectrochemical Analyzers, and the 871 Series conductivity, pH and ORPsensors all commercially available from Invensys Systems, Inc. ofFoxboro, Mass.

As also shown, a temperature sensor 480, such as a conventionalresistance temperature detector (RTD), may be physically coupled to theconduit to detect the temperature of the process fluid, and electricallycoupled to connector 470.

Turning now to FIG. 6, sensor 200 or 400 may be wired by connectingdrive toroids 220, 224 (or 420, 424) to legs A and B of connector 470.Sense toroid 222 (422) may be connected to legs D and E of the connector470. The optional magnetic shields 250, 252 may be connected to leg C ofthe connector. Temperature sensor or thermosensor 480 may be connectedto legs F, G, and H of connector 670.

Calibration conductor 471 extends from terminal A of the test port 676through toroids 620, 622, 624, and returns to terminal B thereof.Optional leak detection conductor 477 (shown in phantom), with orwithout resistor 478, extends from leg C of port 476, intoleak-contacting proximity to the conduit, and in spaced relation fromthe toroids, and returns to leg D of the calibrator.

FIG. 7 shows a wiring schematic of an embodiment substantially similarto those shown and described hereinabove with respect to FIGS. 2 & 4, inwhich the principal flow path 210, and optionally, the secondary flowpath 212, each include one drive toroid and two sense toroids. As shown,drive toroids 720, 734 are connected to terminals A and B of connector470. Sense toroids 722, 724, 730, 732 are connected to legs D, E ofconnector 470. Copper washers 350, 352, 354, 356 serve as magneticshields between the toroids and are grounded at terminal C of connector470. RTD 480 serves as a thermosensing means and is connected toterminals F, G, H of connector 470. Optional leak detection conductor477 (shown in phantom), which may include resistor 478, may be connectedto terminals C and D of test port 476 as shown.

Embodiments of the invention having been described, the operationthereof will be discussed with reference to the following Table I.

TABLE I 802 fasten conduit ends 204 and 206 in process flow line 804couple connector 470 to a data capture device/processor 806 calibrate byshorting terminals A & B of test port 810 activate drive coils 812capture current of sense coils 814 calculate measured conductivity value815 map calculated conductivity value to known conductivity of thecalibration loop 816 disable calibration loop 818 initiate process flow819 repeat steps 810, 812 and 814, to generate conductivity values forthe process fluid. 820 Optionally monitor system for leakage

As shown, conduit ends 204 and 206 are fastened 802 in series with aprocess flow line, and connector 470 is coupled 804 to a data capturedevice/processor such as an analyzer of the type available commerciallyfrom Invensys Systems, Inc., as discussed hereinabove. The sensor maythen be calibrated 806, e.g., using a conventional calibrator coupled totest port 476, which shorts terminals A & B thereof to provide a closedinduction loop of known resistance as described hereinabove. Thereafter,a current may be fed 810 to terminals A & B of connector 470, toactivate the drive coil(s) in parallel with one another, to induce an EMfield in the calibration loop, and in turn, induce a current in thesense coils. Since the sense coil(s) are similarly wired in parallelwith one another, a single current value may be captured 812 atterminals D & E of connector 470. This captured current value may thenbe used in a conventional manner to calculate 814 a measuredconductivity value. The calculated conductivity value is then adjustedor mapped 815 to the known conductivity of the calibration loop. Oncecalibrated, terminals A & B of test port 476 are disconnected 816 fromone another to disable the calibration loop, and process fluid ispermitted to flow 818 through the device. Steps 810, 812 and 814 arethen repeated 819, to generate conductivity values for the processfluid. Optionally, the flow conduit may be monitored 820 for leakage, byperiodically checking for any deviation from baseline resistance of leakdetection conductor 477 and/or resistor 478. As described hereinabove,the use of parallel fluid flow paths provides a completely fluidicinduction loop that eliminates the need for any metallic conductors tocontact the process fluid. This, in turn, enables the conductivitymeasurement of process fluids that are incompatible with metals. Inaddition, the redundancy of drive and/or sense coils serves to enhanceinduction within the fluidic loop for improved measurement sensitivityand/or accuracy.

In the preceding specification, the invention has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications and changes may be made thereunto withoutdeparting from the broader spirit and scope of the invention as setforth in the claims that follow. The specification and drawings areaccordingly to be regarded in an illustrative rather than restrictivesense.

1. An electrodeless conductivity sensor for determining conductivity ofa process fluid, said sensor comprising: a non-electrically conductiveconduit for the flow of a process fluid, said conduit having an inletand an outlet; said conduit diverging downstream of the inlet into firstand second legs, said legs re-converging upstream of said outlet, toform a fluid-flow loop between said inlet and said outlet; at least onefirst toroid configured as a first type coil disposed about one of saidfirst and second legs; at least one second toroid configured as a secondtype coil disposed about one of said first and second legs; at least onethird toroid configured as a redundant one or said first or second typecoils disposed about one of said first and second legs; said first typeand second type coils selected from the group consisting of drive andsense coils; at least one other sense coil disposed about said conduitoutside of said fluid loop; and a connector configured to couple saidfirst, second and third toroids to an analyzer.
 2. The sensor of claim1, wherein said at least one other sense coil is connected electricallyout of phase with sense coils disposed on said first and second legs. 3.An electrodeless conductivity sensor for determining conductivity of aprocess fluid, said sensor comprising: a non-electrically conductiveconduit for the flow of a fluid, said conduit having an inlet and anoutlet; said conduit diverging downstream of the inlet into first andsecond legs, said legs re-converging upstream of said outlet, forming afluid loop between said inlet and said outlet; a housing enclosing saidfirst and second legs; a plurality of toroids of first and second typessurrounding each of said first and second legs; at least one toroid ofsaid first type disposed between toroids of said second type on each ofsaid first and second legs; the first type and second type each beingselected from the group consisting of drive coils and sense coils; atleast one other toroid configured as a sense coil, disposed about saidconduit outside of said fluid loop; wherein said device is configuredfor use in a non-metallic fluid flow system; shields interspersedbetween said coils, configured to magnetically isolate said coils fromone another; a calibration loop including an electrical conductorextending through said plurality of toroids; and a leakage detectorincluding an other electrical conductor disposed within said housing inspaced relation from said plurality of toroids, said leakage detectorconnectable to resistance measuring means.
 4. An apparatus for detectingleakage of process fluid from a fluid flow conduit, the apparatuscomprising: an electrical conductor disposed in leakage-contactingrelation to the conduit; said conductor having a predeterminedelectrical resistance; a test port having terminals coupled to oppositeends of said conductor; said test port being couplable to resistancemeasuring means for measuring resistance of said sensing conductor. 5.The apparatus of claim 4, being integrated with a sensor configured togenerate data relating to a parameter of the process fluid.
 6. Theapparatus of claim 5, wherein the sensor is selected from the groupconsisting of temperature detectors, pressure detectors, densitydetectors, conductivity sensors, pH sensors, ORP sensors, flow meters,and combinations thereof.
 7. The apparatus of claim 4, wherein said testport comprises a calibration port.
 8. The apparatus of claim 4, whereinsaid conductor comprises an electric wire.
 9. The apparatus of claim 4,wherein said conductor is wrapped helically around the conduit.
 10. Theapparatus of claim 4, comprising a resistor disposed electrically inseries with said conductor.
 11. The apparatus of claim 4, furthercomprising a resistance detector coupled to said conductor, saidresistance detector configured to detect any deviation from saidpredetermined resistance.
 12. A method for fabricating a sensor fordetecting conductivity of a fluid flowing through a conduit, said methodcomprising: (a) providing a non-metallic conduit for the flow of aprocess fluid, the conduit having an inlet and an outlet; (b) divergingthe conduit downstream of the inlet into first and second legs; (c)re-converging the legs upstream of the outlet to form a fluid-flow loopbetween the inlet and the outlet; (d) disposing at least one firsttoroid configured as a first type coil about one of the first and secondlegs; (e) disposing at least one second toroid configured as a secondtype coil about one of said first and second legs; (f) disposing atleast one third toroid configured as a redundant one of the first orsecond type coils about one of said first and second legs; (g) selectingthe first and second type coils from the group consisting of drive andsense coils; and (h) configuring a connector to couple the first, secondand third toroids to an analyzer.